The present invention concerns a method and the apparatus for calendering paper and board in the manufacture of coated grades of paper or board.
Paper or board is calendered in order to improve its printability properties. Calendering increases the smoothness and glare of the surface, and in addition, it affects the thickness and bulk (cm3/g) of the material. Changes other than those affecting the surface are usually unintentional, because changes in material thickness are not desired. The unavoidable change in bulk must be adapted to the desired surface quality such that the desired surface quality and bulk as well as the desired final material thickness are obtained. Uncoated material can be subjected to calendering prior to the coating step, or the calendering may be carried out after coating, or at several steps. Many types of calendering methods and apparatuses are available, whereof machine calenders, softcalenders and supercalenders may be mentioned, and as the most recent type of calenders, shoe calenders and belt calenders may be cited. Each type of calender has its individual effect on the quality of the produced material, as well as its own typical field of application. The different types of calenders and their use are well known in the manufacture of paper and board.
European patent No. 0 370 185 describes a typical shoe calender comprising a backing roll and an arched shoe-like stop designed to encircle part of the roll surface. An endless belt travels around the shoe and is fitted to move at the same speed as the material being treated. The material to be calendered travels between the belt and the backing roll and is glazed against the surface of the backing roll. The backing roll can be heated and deformations of the surface occur due to the press power of the shoe and the backing roll, and due to heat. The glazing result is naturally also affected by the wetness of the web. The shoe calender provides a number of advantages, such as the fact that due to the longer dwell time, a smaller compression load and possibly temperature may be used than in roll calenders, still achieving a similar end result. Due to the reduced nip pressure a smaller contraction of the calendered web is achieved, thus preserving a greater part of the original stiffness, or bulk, of the web.
A shoe calender usually provides better glare than the corresponding softcalender.
The German Patent Application No. DE 43 22 876 describes a shoe calender with a smaller shoe width and thus also a shorter glazing zone than in the above-cited solution. In this calender, two calendering nips can be fitted against the same backing roll, and the strap surrounding the shoe is similar to the roll jacket. In German Patent publication No. DE 44 10 129, a shoe calender is described where the shoe is divided into two zones in the moving direction of the web being treated, the press power of the zones against the backing roll being adjustable independent of each other.
Belt calenders, in which the calender zone is provided by a roll and a belt pressed against said roll by means of a second roll, bear a close resemblance to shoe calenders. The belt may be flexible in the direction of its thickness, whereby a pressure treatment zone defined by the properties of the belt and the geometry and loading force of the calender rolls used will be provided between the backing roll and the belt. Also a calender having a very short pressing shoe is termed a belt calender, whereby the length of the nip almost corresponds to a nip formed with two rolls.
Long-nip calenders, belt calenders and softcalenders are well suited for calendering board. In PCT patent publication No. WO 96/28609, a coated packing board is described, whose manufacturing process involves the use of a lengthened soft calender nip. The board is calendered after coating. By calendering, a sufficiently good printing surface is obtained, and due to the lengthened nip, a lower pressure may be applied during calendering, whereby a smaller reduction in density and basis weight is achieved. This is of particular advantage in the manufacture of packing board because a lighter board provides greater flexural strength. The length of the lengthened nip of the calender used is reported as being from 30 to 100 mm, preferably 60 to 70 mm. Thus, the calender used is still one having a fairly short nip. The web speed and dwell time used are not reported.
In PCT patent publication No. WO 97/44524, a method for manufacturing LWC paper is described. This publication makes clear the considerable effect of the treatment temperature on the properties of the paper being manufactured. According to the publication, coated paper is calendered in a soft calender and the temperature of the paper is kept below the softening temperature of lignin. According to the publication, the method achieves much better glare than previously known methods involving the use of a softcalender.
The present invention aims at providing a method for manufacturing precalendered and end-calendered coated paper or board.
The invention is based on first calendering the material to be treated in a long-nip calender, and after coating, in a calender having a short nip.
The invention achieves considerable benefits.
Paper and board which consist of plant fibers behave in different ways during calendering when uncoated and when coated. In addition, the calendering is essentially affected by the moisture content of the material, wherefore calendering is usually accompanied by moisture control of the treated material. By means of the invention, these material properties can be exploited, thereby achieving paper and especially board having better printability and strength properties than previously obtained. As regards board and also the thickest grades of paper, a change in density during calendering essentially affects the strength properties of the material in the above-described manner, wherefore it is of particular advantage to be able to control the calendering process during the different manufacturing steps in accordance with the material properties when producing these materials.